1. Field of the Invention
The present invention relates to a conjugated molecular assembly, a method of fabricating the assembly, and a device including the assembly and, in particular, a conjugated molecular assembly having an extended conjugated segment.
2. Description of the Related Art
During the past three decades, renewed interest and considerable progress has been made in the synthesis and chemical and physical understanding of conjugated molecules and macromolecules. The attention is largely driven by applications of conjugated molecular assemblies in optical, optoelectronic, electronic, and sensor applications.
Most conjugated molecular assemblies include small molecules or short-chain oligomers, less than 8 monomeric units long. These small molecules may be found in nature, readily prepared by common organic synthetic techniques, and manipulated in organic solvents.
The nature of the π-bonding that gives rise to conjugation and the interesting physical properties is also the source of molecular rigidity and insolubility. As the size of conjugated molecules increases, they become increasingly less soluble. For example, in the homologous oligothiophene series quaterthiophene (4 thiophene monomers) is soluble, but sexithiophene (6 thiophene monomers) is not soluble. The insolubility of these assemblies can in some cases be overcome by chemically functionalizing the end or sides of conjugated molecules, but this also may change the desirable physical properties of the molecules and their assemblies.
Very long molecules (e.g., greater than 20 monomers long) are typically classified as polymers. Polymers are polydisperse and characterized by their average molecular weight. Polymer chains consist of a distribution of shorter, rigid molecular segments. This gives rise, especially when functionalized, to solubility not found in shorter molecules and oligomers, but also to a distribution in physical properties and a lack of order in polymeric thin films.
Recently, sequential synthetic routes to prepare well-defined molecules and macromolecules has been demonstrated [J. M. Tour, “Conjugated Macromolecules of Precise Length and Constitution. Organic Synthesis for the Construction of Nanoarchitectures,” Chem. Rev. 96, 537 (1996), P. R. L. Malenfant, J. M. J. Frechet, “The first solid-phase synthesis of oligothiophenes,” Chem. Comm., 23, 2657 (1998).) by solution-phase chemical techniques and on resins (solid supports). While some assemblies have been synthesized, these techniques do not enable assembly onto surfaces.
Chemical functionalization of short molecules with head groups that enable the binding to and the self-assembly of molecules onto surfaces has received enormous attention. Once assembled onto a surface these molecular layers are known as self-assembled monolayers (SAMs). These molecules are typically synthesized and then allowed to self-assemble from common solvents onto surfaces. Self-assembled monolayers have received tremendous attention as etch barriers, surface layers to alter surface chemistry, and resist layers.
Conjugated SAMs have received particular interest as active layers in molecular electronic and photonic devices and have more recently been discussed as the active layers in molecular sensors. The SAMs investigated to date are typically short conjugated molecular segments as the longer molecules, which may be interesting for devices, are no longer soluble and therefore have not been investigated. Molecular electronic and memory devices are based on intramolecular charge transport and building the desired functionality, such as switching, within the molecule. Molecular sensors may also be based on changes in intramolecular charge transport arising from the detection of an analyte by a receptor that is a part of the molecule. A molecular scale transistor is a necessary component for logic applications and has often been referred to as the holy grail of molecular electronics.
To build a molecular scale transistor is no small feat. The channel length of a molecular transistor is defined by the length of the molecule. Similarly, the molecular switching medium in a memory cell or the functionalized molecule in a molecular sensor defines the interelectrode distance. In order to attain a suitable “off-state” of a molecular device in both transistors and memory cells and a measurable change in resistance in a sensor, the tunneling current between electrodes (source and drain electrodes in a transistor) must be suppressed. This places a lower limit on the channel length or cell height in transistors and memory cells, respectively, and therefore the length of the molecule at about 2.5 nm–3.0 nm [C. R. Kagan, A. Afzali, R. Martel, L. M. Gignac, P. M. Solomon, A. G. Schrott, B. Ek, “Evaluations and Considerations for Self-Assembled Monolayer Field-Effect Transistors,” Nano Letters, 3, 119 (2003)]. The lower limit on the channel length and the length of the molecule is also constrained in a molecular field-effect transistor by electrostatics and the requirement to introduce a sufficient gate field to modulate the conductance of the molecular channel. This may also be true in a chemical field-effect transistor for sensing. The self-assembled conjugated molecules that have been investigated are typically shorter than 2.5 nm–3.0 nm.
The desired functionality of a molecular device may necessitate the incorporation of different molecular segments. The long-term vision of building the entire functionality of a molecular device within the molecule may require the introduction of different molecular species with different chemical and physical function. To date, only very simple and short conjugated self-assembled molecules have been investigated and therefore have not incorporated some of the desired molecular functionality.
Layer-by-layer assembly techniques have been used to prepare longer length scale molecular assemblies on surfaces. One of the most developed assemblies grown layer-by-layer is the layered metal phosphates and phosphonates. The films include multivalent metal ions, e.g. Zr4+, and organic molecules terminated with an acidic functionality, such as a phosphonic acid [e.g., see Cao, Hong, Mallouk, “Layered Metal Phosphates and Phosphonates: From Crystals to Monolayers” Acc. Chem. Res., 25, 420 (1992)].
Katz and co-workers have used this method to align hyperpolarizable molecules into polar multilayer films that show second-order nonlinear optical effects (e.g., see U.S. Pat. Nos. 5,217,792 and 5,326,626). A similar approach has also been extended to other materials such as polymers, natural proteins, colloids, and inorganic clusters [e.g., see Decher, “Fuzzy Nanoassemblies: Toward Layered Polymer Multicomposites,” Science 277,1232 (1997)]. This same technique has also been applied to the production of other multilayers including Co-diisocyanide, dithiols with Cu, and pyrazines with Ru [e.g., see Page, Langmuir, “Coordinate Covalent Cobalt-Diisocyanide Multilayer Thin Films Grown One Molecular Layer at a Time” 16, 1172, (2000)].
Among the existing examples, the driving force for the film progression is mainly the electrostatic interactions between polycations and polyanions. Few examples involve other types of interactions, such as hydrogen bond, covalent, or mixed covalent-ionic. Recently the inventors demonstrated the use of strong covalent interactions, rather than ionic interactions, between metals and ligands in a novel strategy to assemble nearly perfectly packed multilayers of metal-metal bonded supramolecules and utilized this approach in molecular devices (YOR920010784US1, YOR920020094US1, C. Lin, C. R. Kagan, “Layer-by-Layer Growth of Metal-Metal Bonded Supramolecular Thin Films and Its Use in the Fabrication of Lateral Nanoscale Devices,” J. Am. Chem. Soc., 125, 336 (2003)). While there are a few examples of layer-by-layer assembly using covalent interactions, none of the examples allow for carbon-carbon bond formation.
To harness the optical, optoelectronic, electronic, and sensor properties of conjugated molecular assemblies in solid-state applications and devices, development of new methods for incorporating molecules with longer molecular lengths and with a wide range of functionality are needed. Layer-by-layer assembly methods, while attractive for the formation of extended molecular structures, have not been demonstrated for the formation of carbon-carbon bonds necessary to form the extended conjugated molecular assemblies interesting for applications.